1. Field of the Invention
The invention relates to injection molding systems and specifically to a heater for hot runner nozzles for such systems.
2. Related Prior Art
Common to all devices for processing thermoplastic polymeric resins are associated means for obtaining and maintaining thermoplasticity of the resin during processing, such as the shaping of the resin material into an article. Maintenance of thermoplasticity may require both heating and cooling of the resin at various times in the processing of the resin. Cooling may be needed to avoid thermal degradation of the resin. Almost all of the resin processing techniques rely at least in part on heating or cooling of the polymeric resin by thermal transfer through the agency of a metal surface, part of the processing apparatus. Heat is generally applied to an outside surface of the metal apparatus by concentrated heat sources such as band heaters, or from within the body of the metal part by heater rods or circulating, heated fluids such as air, water or other chemical liquids. In all cases, the metal heat transferring components have to be of substantial thickness and mass to resist extreme pressures and mechanical forces. The large metal mass responds slowly to changes in thermal input or cooling so that precise control of narrow temperature ranges is difficult to control. Also, when temperature differences are desired in adjacent zones of the same apparatus, it is difficult to localize the particular and different temperatures to be maintained for appreciable periods of time. This shortcoming is especially troublesome for relatively complex processing techniques and apparatus, such as in the injection molding of large parts.
Hot runner injection molding systems have several melted material flow passageways that are uniformly heated over the entire flow path leading from a molten reservoir to a mold cavity or cold runner. The melted material that flows through the passageway must remain liquid until reaching the mold cavity or cold runner. To control flow rate and pressure, the heated passageway leads to or from injection mold runner nozzles which may be externally heated. This nozzle is sometimes referred to as a hot runner gate injection nozzle or a hot runner probe injection nozzle but will hereafter be simply referred to as a “runner nozzle.” These runner nozzles are typically located in the hot runner molding system's manifold base. The nozzles extend through ports leading to each of the molding cavities or to a secondary heated or unheated passageway within a mold block. It is essential to adequately and uniformly heat the runner nozzle because this is often the final point in the heated portion of the flow passageway just prior to being injected into the mold. At this point the material must be at or above its melting point in order for the molten material to flow freely through the runner nozzle, so the nozzle can reliably perform its function of controlling flow rate.
Significant transitions in temperature at the point of the runner nozzle are not desirable as the nozzle is a key part of any molding process because transitions in temperature may change the fluid consistency of the melted material such as thermoplastic which may result in a defective final product. Also, if it is desired to intermittently shut off flow and turn flow back on for a given nozzle, heating of the nozzle is necessary to maintain the residual material in a melted state, to prevent clogging.
Currently, runner nozzles are typically heated by a heat source external to the nozzle. Typically, the runner nozzle is heated by a resistive wire proportionally spirally wound heating element. The spirally wound element forms a cylinder that is co-axially disposed about the exterior surface of the runner nozzle. However this type of heater configuration operates inefficiently due to heat loss because of the open exposure of the heating element to the surrounding environment. It also increases the diameter of the nozzle and thus requires bigger openings in the manifold plate to receive the nozzle. Also, many of the standard nozzle heaters are not completely encapsulated by an insulated sheath, which make it more difficult to maintain a temperature at the runner nozzle location that is uniform with the remainder of the flow passageway. In addition the physical design of the resistive element (i.e. spiral) is limited as well. The gauge of the resistive wire heating element required to generate enough heat is such that the wire cannot be formed into complex circuit patterns. In many cases various complex circuit patterns other than a simple spiral pattern are desired in order to achieve more efficient heat distribution. Also, these types of heaters can be bulky and difficult to maintain and repair. Installation is difficult because of the large leads of the resistive element, and the mold designer must allocate space for the large leads and increased nozzle/heater combination. In addition, in many cases the externally heated runner nozzle apparatus has to be adapted to accommodate a thermocouple device which requires an additional space for the thermocouple and its wiring. A better way is needed to uniformly heat the runner nozzle, heat it efficiently and the design should be cost effective and easy to maintain and repair.
Conventional industrial equipment which provides heat externally to a flow passage, such as the subject runner nozzle, will generally provide heat by the means described above or by a single or multiple band heater design.
In U.S. Pat. No. 5,973,296 to Juliano, et al., the invention is a tubular heater that consists of a metallic tubular substrate that has a dielectric film layer and a resistive thick film layer applied directly to the exterior cylindrical surface of a tubular substrate by the method of precision fine film printing. This method is similar to the method used to produce some thick-film resistors. The precision fine film printing requires the use of an expensive fine film printing machine that uses a fine tip writing pen to dispense the conductive ink.
U.S. Pat. No. 5,411,392 to Von Buren, teaches a slotted band heater in conjunction with a slotted clamping sleeve that installs over a hot runner nozzle. This two part device utilizes the clamping force of the outer sleeve to maintain thermal communication between the band heater and the nozzle.
In U.S. Pat. No. 4,922,082 to Bredt et al., an electrical resistive heating device which comprises two co-axially spaced apart electrodes, each in intimate surface-to-surface contact with an interposed heating element is disclosed. The heating element comprises a powdered material which functions as an electrical resistive heater when an electrical potential difference is applied thereacross by the electrodes. Heat generated in such element is conducted through at least one of the electrodes which in turn conducts it to an object which it is desired to heat. The powdered material is pressed in the annular space between the two electrodes.
A new heating device is needed that can be easily installed over a runner nozzle or other conduit and be readily massed produced, reliable, provide repeatable and predictable temperature profiles at a reduced manufacturing and maintenance cost.
The improved apparatus of the present invention includes as a heater means a multi-layer thick-film heater which may be mounted in close association with the thermoplastic polymeric resins being processed in the apparatus. Heavy metal components to achieve thermal transfer to the resin are not necessary. There can be a saving of weight, materials and labor in manufacture. With the closer juxtaposition of the heating element in the subject plastic, a closer control of resin temperature is maintainable with quicker response times to maintain a pre-determined resin temperature, even in adjacent but different zones or localities. The lower thermal mass of the heating elements is more responsive to cooling or changes from heating to cooling or cooling to heating. A more accurate and repeatable temperature profile can be obtained with the device resulting in improved machine performance and a higher quality finished product.